High voltage cable with potential gradient equalization means



Nov. 19, 1968 LARS'GORAN VIRSBERG ET AL HIGH VOLTAGE CABLE WITHPOTENTIAL GRADIENT EQUALIZATION MEANS Original Filed May 2. 1966INVENTDR. 0am! Vmsme oRflu lIwRsLL inns Leas-6 United States Patent3,412,200 HIGH VOLTAGE CABLE WITH POTENTIAL GRADIENT EQUALIZATION MEANSLars-Goran Virsberg, Vasteras, and Lars-Goran Laurell,

Stockholm, Sweden, assignors to Allmanna Svenska ElektriskaAktiebolaget, Vasteras, Sweden Continuation of application Ser. No.546,673, May 2,

1966. This application Dec. 8, 1966, Ser. No. 600,273

Claims. (Cl. 174-102) ABSTRACT OF THE DISCLOSURE A high voltage meanscomprising a cable with an electrical insulation applied around theconductor and a body of conducting material surrounding the insulationand having a potential considerably different from that of theconductor. The body of conducting material leaves a portion of theinsulation exposed outside an edge of the body. A coating of a materialhaving a substantially voltage dependent resistivity is applied to thesurface of the exposed portion of the insulation and in electricalcontact with the body of conducting material. The coating comprises awrapping of a tape comprising a binding material selected from the groupconsisting of thermoplastics and elastomers and silicon carbideparticles intermixed in the binding material.

This application is a continuation of application S.N. 546,673, filedMay 2, 1966, now abandoned, which is a continuation of application, S.N.271,726, filed Apr. 9, 1963, now abandoned.

The present invention relates to a coating having a pronounced voltagedependent resistivity for equalizing the potential gradient along thesurface of an electrical insulation, particularly on the surface of anexposed insulation outside a screen edge of a cable as described in theco-pending application Ser. No. 271,726, of which the presentapplication is a continuation.

In the US. Patent No. 3,066,180 is described a method of equalizing thepotential gradient along the surface of an electrical insulation, whichconsists in the surface being provided with a coating containingconstituents which give the coating a pronounced voltage dependentresistivity. Such a coating has a strong non-linear currentvoltagecharacteristic so that the voltage over the active part ofthe coating,within a Wide voltage range, becomes substantially constant andindependent of the current intensity in that part of the coating. Theresistivity of the coatings automatically assumes the most suitablevalue in each part independent of the potential of the conductor, sothat the coatings automatically achieve satisfactory equalization of thepotential gradient even if the voltage of the conductor varies withinwide limits.

In the said US. patent a thermosetting varnish containing siliconcarbide is mentioned as an example of a material from which the coatingcan be made. -It is stated that this varnish could either be applieddirectly on to the surface of the insulation or could be applied firston a tape, for example of fibrous material, which is thereafter wound onto the insulation of the conductor, and the varnish subsequently cured.

Especially when the method described in the US. Patent No. 3,066,180 isused for equalizing the potential gradient along the surface of aninsulation exposed outside a screen edge of a cable, certain drawbacksare involved in producing the coating with a thermosetting varnish. Onedrawback is that the cable cannot be mounted immediately the varnish hasbeen applied since the varnish must dry which, particularly if noheating device is "ice available, may take a considerable time. Sincejoining and connecting of the cables often takes place out of doors,this drawback has considerable importance. Of course, on such occasionsworking with a fluid product and the inconvenience this involves whenapplying the product, is also associated with disadvantages, inter alia,with sedimentation problems. Apart from whether the varnish is applieddirectly on the surface of the insulation or first on a tape which isafterwards wound on to the insulation of the conductor, there arefurther problems 'with storing the product. This is particularly so ifthe varnish has a short curing time since its storing time will then beshort.

The above-mentioned disadvantages in applying a coating having voltagedependent resistivity on the surface of an insulation, e.g. aninsulation exposed outside a screen edge of a cable, are completelyavoided according to the present invention. According to the inventionthe coating comprises .a wrapping of a tape applied around theinsulation and comprising a thermoplastic or an elastomer, for examplepolyvinyl chloride, and silicon carbide mixed in the thermoplastic orelastomer, which gives the tape a pronounced voltage dependentresistivity. By a pronounced voltage dependent resistivity is meant thatthe exponent a in the equation I =C U has a value of at least about 2,where -I is the current intensity, U the voltage and C a constant.

The tapes according to the invention are flexible, dry and may be storedindefinitely. They can without difficulty and without auxiliary means beapplied on the insulation to a coating in the form of a tight, wellfitting casing. Immediately after they have been applied, mounting ofthe cable may be finalized.

The thermoplastic or elastomer in the tapes may, besides the mentionedpolyvinyl chloride, consist of, inter alia, polyethylene, polybutylene,polypropylene, ethyl cellulose, polyamide, natural or synthetic rubber,such as natural rubber NR, styrene-butadiene-rubber SBR (e.g. type 1502according to ASTM), chloroprene rubber CR, neoprene GN-A, nitrite rubberNBR, chlorosulphone rubber Hypalon, butyl rubber IIR (e.g. Esso Butyl325), stereo isoprene rubber IR (e.g. Shell IR- rubber 305), stereobutadiene rubber BR (e.g. Ameripol CB), poly-urethane rubber.

The silicon carbide may be made of a quality which is normally calledelectrocarbide and is used in nonlinear components, for example inlightning arresters and varistors. This quality has relatively lowvolume resistivity. It can also be made of a quality with high volumeresistivity. By the expression silicon carbide with high volumeresistivity is meant silicon carbide types 'which are electronicconductors usually on n-conducting type and have a volume resistivity ofbetween a few ohm-cm. to several million ohm-cm. The last-mentioned typeof silicon carbide is usually green and is normally used as abrasive.Silicon carbide with high volume resistivity has been found particularlysuitable for use since it is then possible to use such a high carbidepercentage that the occurrence of carbide grain insulated from eachother is avoided and thus also the risk of corona within the coating atpoints with high local field intensity. By using particles withirregular shape, e.g. particles obtained through crushing, specialeffects are attained which do not appear when using round particles.

The percentage of silicon carbide in the tape is suitably about lO-about60, preferably 25-50 percent by volume. The particle size of the siliconcarbide is suitably smaller than particle size and preferably in therange-particle size 600 to particle size 200. These particle sizes andother particle sizes specified in the specifications are according toUS. Standard Sieve Specifications. The carbide may consist of a singleparticle size or of mixtures of two or several particle sizes.

The tape may be manufactured in such a way that a warm mouldable mixtureof the thermoplastic or the elastomer and the silicon carbide particlesis shaped to a sheet-' shaped product by passing it through a rollingmeans with at least one pair of rolls comprising a first and a secondrotating hot roll and that the shaped sheet is taken up on a coolingpath in the form of a movable carrying support for the sheet, e.g. anendless movable mat or rubber, the cooling path being arranged close tothe rolling means and to move with a speed equal to the speed at whichthe sheet is fed from the pair of rolls. It is suitable that the shapedsheet is brought to move with the second roll while lying against itsenvelope surface during a part of a revolution before it is taken up onthe cooling path, which is arranged to lie close to the envelope surfaceof this roll and to move with the same speed as the peripheric speed ofthis roll.

According to an advantageous embodiment the shaped sheet is brought tomove with the second roll while lying against its envelope surfaceduring a part of a revolution, after which it is transferred to a thirdrotating roll, drum or the like, arranged with a small radial distanceto the second roll, drum or the like, arranged with a small radialdistance to the second roll, the third roll being kept at a lowertemperature than the first and second roll and being brought to movewith the same periphery speed as the second roll. The shaped sheet isthen brought to move with the third roll while lying against itsenvelope surface during a part of a revolution, before it is taken up onthe cooling path, which is arranged to lie close to the envelope surfaceof the third roll and to move with the same speed as the periphery speedof the third roll. By arranging in the described way the third rollserving as cooling roll, the result is obtained that the shaped sheetdoes not have any tendency to stick to the cooling path. This is ofcourse of particular importance at continuous manufacture of sheets.

The invention will be explained more closely with reference to thedescription of some embodiments chosen as examples which are shown inthe accompanying drawing, where FIGURE 1 shows a cable comprising asingle conductor, FIGURE 2 one comprising three conductors, and FIGURE 3an arrangement for manufacturing tapes used according to the invention.

In accordance with the FIGURES 1 and 2 the metallic conductor 1 isprovided with an insulation 2 which may consist, for example, ofplasticized polyvinyl chloride or polyethylene, but also, inter alia, ofpaper. The insulated conductor has outside the conductor insulation asemiconducting layer 3 which forms an equipotential surface on theoutside of the insulated conductor and may consist, for example, of athermoplastic or an elastomer containing frame building carbon black, orof a graphitized paper. The thermoplastic or elastomer may be formed,inter alia, from polyvinyl chloride, polyethylene, natural or syntheticrubber. The layer may consist of a seamless extruded casing or ofspirally running tapes. If the cable has several conductors as in FIGURE2 these are combined into one cable and provided with a commonsemiconducting casing 4 which may consist of the same material and bemanufactured in the same way as the semiconducting layers on theindividual insulated conductors. On top of the layer 3 in the singleconductor cable or the common casing 4 in the multi-conductor cable is ametallic sheath 5 which may consist of threads or tapes applied inspiral form or longitudinally applied tapes or seamlessly applied metalsuch as lead or aluminum. The metal sheath may be outwardly protected bya casing 6 of, for example, polyvinyl chloride.

When a cable of the above described type is being mounted, the metallicscreen 5 is earthed so that the semiconducting layer 3 of the insulatedconductors receives earth potential either by direct contact with themetal screen as is the case with the single conductor cable according toFIGURE 1 or by contact with the metal screen via the common casing 4, asis the case with the multi-conductor cable according to FIGURE 2. Whenmounting, the cable ends are scaled and the individual insulatedconductors freed as shown in the FIGURES l and 2. In order to equalizethe field concentration which arises during testing and operation at thescreen edge, according to the invention the insulated part 2 is providedwith a coating having a pronounced voltage dependent resistivity, in theform of a wrapping 7 of a tape of a thermoplastic or elastomer, forexample polyvinyl chloride, containing silicon carbide. Examples ofsuitable compositions for the material in the tape are given later on inthe description. In the embodiments shown in the figures the coatingconsists of a tape which is wound around the insulation of eachconductor with overlap, for example, 50% overlap. In accordance wih thefigures, the coating 7 overlaps the bandage 3 of the semi-conductingtape. The length of the coating is dependent upon the stress which thecable is intended to withstand. Normally a length of a few cm. issufficient.

In certain cases it may be suitable to allow the coating 7 and thesemi-conducting layer 3, whether the cable is a single conductor cableas in FIGURE 2 or a multiconductor cable as in FIGURE 2, to consist ofone and the same tape, whereby thus the coating 7 constitutes acontinuation of the layer 3 situated within the sheath.

In the following, some examples are given for suitable compositions forthe material in the thermoplastic or elastomer tape.

EXAMPLE 1 A mixture consisting of 67 parts by weight polyvinyl chloride,40 parts by weight dioctyl phthalate, 150 parts by Weight 400 particlesize silicon carbide with high volume resistivity, 3 parts by weight ofthe lead compound 3PhO.PbSO .I-I O and 0.5 part by weight stearic acidare premixed in a ribbon mixer. The percentage by volume of siliconcarbide in this thermoplastic mixture is about 33. This powder mixtureis then kneaded and gelatined in an internal mixer, e.g. of Banbury typefor 5 to 7 minutes with an end and emptying temperature of 145- 150 C.The ready-mixed mixture is then transferred to a two-roller mill, therolls of which have a temperature of 150-160 C., where it is furtherWorked for about 5 minutes. From the roller mill the mixture is thensuccessively cut off to be fed to a calender machine. The mentionedabout 150 C. heat gelatined mixture, which is designated 10 in FIGURE 3is fed into the calender machine 13 between the rolls 11 and 12. Therolls have the same periphery speed and are adjusted so that they give asheet with the desired thickness. The roll 11 has a temperature of about110-115" 'C. and the roll 12 a temperature of about 125-135 C. The sheet14 formed by the passage of the pair of rolls follows the rolls 12 andis transferred from this to a cold roll 15. This roll has the sameperiphery speed as the other rolls and a temperature lower than 50 C.,e.g. 20-25 C. It serves as a cooling roll. The distance between themiddle roll 12 and the lowest roll 15 is so great that the rolled sheetgoes clear between these rolls. Under the lowest roll there is a coolingpath 16 in the form of an endless rubber mat or an endless steel belt orthe like which follows a part of the envelope surface of the roll andwhich is driven by a roller 17 and goes around the rollers 18 and 19.The cooling path is driven with the same speed as the periphery speed ofthe lowest roll 15. The sheet is taken up and transported by the coolingpath. After passage of the rollers 20, 21 and 22, the sheet is wound upon to the Winding device 23.

Under certain circumstances it is possible to make the sheet without theuse of the lowest roll 15. The cooling path 16 is then arranged on theunder side of the roll 12. This can be made analogously in the mannershown in FIGURE 3 but with respect to the opposite feeding direction ofthis roll, the cooling path has to be turned so that its movementdirection is opposite to that shown in FIGURE 3.

The manufactured sheet may be cut in its longitudinal direction to tapes7 with desired width. The tape 7 in FIGURES 1 and 2 may for example havea width of about 20 mm. and a thickness of about 0.3 mm. and may beapplied with for example 75% overlap in accordance with the figures onto a 10 kv. high voltage cable provided wtih plastic insulation so thatthe length of the coating is about 7 cm.

EXAMPLE 2 In the same way and using the same material given in Example1, with the exception that instead of silicon carbide with high volumeresistivity, electroca-rbide" is used, a tape is produced which is thenapplied in the way illustrated in Example 1.

EXAMPLE 3 67 parts by weight polyvinyl chloride, 33 parts by weightdiocytl phthalate, 3 parts by weight 3 PbO'PbSO H and 100 parts byweight 230 particle size silicon carbide having high volume resistivityare mixed together. The percentage by volume of the silicon carbide inthis thermoplastic mixture is about 27. In the same way as described inExample 1, a tape is produced which is then applied similarly to thedescribed method in Example 1.

EXAMPLE 4 In the same way and using the same material as mentioned inExample 3, with the exception that instead of silicon carbide havinghigh volume resistivity electrocarbide is used, a tape is produced whichis then applied in the way illustrated in Example 1.

EXAMPLE 5 67 parts by weight polyvinyl chloride, 33 parts by weightdioctyl phthalate, 3 parts by weight 3 PbO-PbSO -H O and 150 parts byweight 600 particle size silicon carbide having high volume resistivityare mixed together. The percentage by volume of silicon carbide in thisthermoplastic mixture is about 38. In the same way as described inExample 1 a tape is produced which is then applied in a similar way tothat described in Example 1.

EXAMPLE 6 In the same way and using the same material as mentioned inExample 5, with the exception that instead of silicon carbide havinghigh volume resistivity electrocarbide is used, a tape is produced whichis then applied in the way illustrated in Example 1.

EXAMPLE 7 67 parts by weight polyvinyl chloride, 33 parts by weightdioctyl phthalate, 3 parts by weight 4 PhD- PbSO -H O and 200 parts byweight 400 particle size silicon carbide having high volume resistivityare mixed together. The percentage by volume of silicon carbide in thisthermoplastic mixture is about 47. In the same way as described inExample 1 a tape is produced which is then applied in a similar way tothe method decsribed in Example 1.

EXAMPLE 8 In the same way and using the same material as mentioned inExample 7, with the exception that instead of silicon carbide havinghigh volume resistivity electrocarbide is used, a tape is produced whichis then applied in the way illustrated in Example 1.

EXAMPLE 9 100 parts by weight Hypalon (E. I. du Pont de Nemours and Co.,U.S.A.) is kneaded in an internal mixter, e.g. of Banbury type for about4 minutes, the temperature then being raised due to developed frictionheat, after which 150 parts by weight 400 particle size silicon carbidewith high volume restivity is added and the mixture kneaded for about afurther 4-6 minutes. The percentage by volume of silicon carbide in thismixture is about 36. The mixture is then transferred to a two-rollermill, the rolls of which have a temperature of between 60-80 C. From theroller the mixture is then succesfully cut off to be fed to the calendermachine 13 according to FIGURE 3 and a sheet is produced as described inExample 1. However, the temperature of the rolls 11 and 12 is in thiscase lower, suitably 50-70 C. The sheet is then as described in Example1 cut up into tapes with desired width and the tapes applied in the waydescribed in Example 1.

EXAMPLE 10 In the same way and using the same material given in Example9, with the exception that instead of silicon carbide with high volumeresistivity, electrocarbide is used, a tape is produced which is thenapplied in the way illustrated in Example 1.

EXAMPLE 11 In the same way and using the material given in Example 9,with the exception that instead of parts by weight silicon carbide 200parts by weight silicon carbide is used, a tape is produced when is thenapplied in the way illustrated in Example 1. The percentage by volume ofsilicon carbide in the tape is about 43.

EXAMPLE 12 In the same way and using the material given in Example 10,with the exception that instead of 150 parts by weight silicon carbide200 parts by Weight silicon carbide is used, a tape is produced which isthen applied in the way illustrated in Example 1. The percentage byvolume of silicon carbide in the tape is about 43.

Any one of the rubbers listed in col. 2 together with Hypalon can beused instead of Hypalon in the Examples 9-12 for manufacturing the tapeunder the conditions stated in these examples and the tape produced canbe applied as described in Example 1.

It should be clear that the invention can with advantage be utilized notonly for an insulation applied outside a screen in a cable, but also inseveral other cases where it is necessary to equalize the potentialgradient on the surface of an insulation such as, for example at thefree coil ends of electrical machines and with high tension bushings.

We claim:

1. A high voltage means comprising a cable with at least one electricalconductor, an electrical insulation applied around said conductor and incontact with said conductor, a body of conducting material surroundingsaid insulation at least partially along its circumference and having apotential considerably different from that of the conductor, said bodyleaving a portion of said insulation exposed outside an edge of saidbody in the axial direction of the conductor, and a coating of amaterial having a substantial voltage dependent resistivity within theactual voltage range of the conductor applied to the surface of theexposed portion of said insulation and in contact with the surface ofthe exposed portion of said insulation, and in electrical contact withsaid body, said coating comprising a wrapping of a tape applied aroundthe exposed portion of said insulation, said wrapping having one endelectrically connected to said body and positioned substantially in theneighborhood of the edge of said body and another end positioned on theexposed portion of said insulation and said tape comprising a bindingmaterial selected from the group consisting of thermoplastics andelastomers and silicon carbide particles intermixed in the bindingmaterial.

2. A high voltage means as claimed in claim 1, in which the body ofconducting material includes a metallic screen.

3. A high voltage means as claimed in claim 1, said silicon carbideparticles having a volume resistivity in the range of a few ohm-cm. toseveral million ohm-cm.

4. A high voltage means as claimed in claim 1, the content of saidsilicon carbide particles being 10-65 percent by volume of said tape.

5. A high voltage means as claimed in claim 1, said silicon carbideparticles having a particle size smaller than particle size 100.

6..A high voltage means as claimed in claim 1, the

content of said silicon carbide particles being 25-50 percent by volume,said silicon carbide particles having a particle size in the range ofparticle size 200 to particle size 600 and said binding materialcomprising polyvinyl chloride. 7. A high voltage means as claimed inclaim 1, said coating comprising a wrapping of layers of said tapeapplied. around the exposed part of the insulation of said conductor inoverlap arrangement.

8. A corona preventive tape comprising a binding material selected fromthe group consisting of thermoplastics and elastomers and intermixed inthe binding material silicon carbide particles, the content of saidsilicon carbide particles being 1065 percent by volume of said tape,said silicon carbide particles having a particle size smaller thanparticle size 100.

References Cited UNITED STATES PATENTS 2,081,517 4/1937 Van Hofl'en174--106 X 2,234,068 3/1941 Wiseman 174-106 X 2,622,152 12/1952 Rosch1741()2 X 2,789,154 4/1957 Peterson 174-73 2,945,913 7/1960 Conangla17473 3,066,180 11/1962 Virsberg 174-73 X 3,047,448 7/1962 Feller174-120 X LEWIS H. MYERS, Primary Examiner.

ELLIOT A. GOLDBERG, Assistant Examiner.

